selftests/bpf: validate fake register spill/fill precision backtracking logic

selftests/bpf: validate fake register spill/fill precision backtracking logic

Add two tests validating that verifier's precision backtracking logic
handles BPF_ST_MEM instructions that produce fake register spill into
register slot. This is happening when non-zero constant is written
directly to a slot, e.g., *(u64 *)(r10 -8) = 123.

Add both full 64-bit register spill, as well as 32-bit "sub-spill".

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231209010958.66758-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>